1.Their liches that had their bodies very heavily damaged, and managed to survive more as a ghost than a walking corpse through uncertain methods. This came at a cost in terms of power and possibly intelligence. (If this is the case, I wonder if their souls are more vulnerable to being recycled by the dreamsleeve?)
2. Their an undead creature created by a Lich from ordinary mages. Most of the power of a lich, but completely obedient.
Or perhaps a Necromancer's failed attempted at becoming one.
Though I like my idea that liches can exist beyond the physical form, pehaps if they have tied their soul to a totem.